This invention relates to a process for the selective production [S,S]-ethylenediamine-N,N'-disuccinic acid.
Ethylenediamine N,N'-disuccinic acid (EDDS) and its various alkali metal, alkaline earth metal, ammonium and substituted ammonium salts are well recognized by the detergent industry as useful chelating agents in cleaning formulations. (See U.S. Pat. No. 4,704,233, which is incorporated herein by reference as if fully set forth.) These salts and acids are theorized to chelate metals such as iron, manganese, copper and other multivalent metal ions. The metal ions are constituents of certain organic stains or act to stabilize such stains when present in washing solutions. Besides providing for the chelating function, EDDS and its salts are non-phosphorous compounds and, as a result, are environmentally desirable. Even further, EDDS and its salts exhibit biodegradability. The degree of biodegradability depends upon the optical EDDS isomer involved. Of the three optical isomers, [R,R], [R,S] and [S,S], the [S,S] isomer is most easily biodegradable and is thus preferred.
The [S,S] isomer can be synthesized from L-aspartic acid salt and 1,2-dibromoethane. A particularly attractive route features reacting sodium L-aspartate and 1,2-dibromoethane in an aqueous medium to yield, in solution, the sodium salts of [S,S] EDDS. See Neal and Rose, Stereospecific Ligands and Their Complexes of Ethylenediamine-disuccinic Acid, Inorganic Chemistry, Vol. 7. (1968), pp. 2405-2412. The Neal and Rose process reacts a fairly high percentage, say about 80%, of the L-aspartate in producing the sodium salt of [S,S] EDDS. This high conversion, however, does not translate into high selectivity for the [S,S] EDDS salt as the process produces a substantial amount of by-products. Thus, the ultimate recovered yield of [S,S] EDDS will be fairly low, e.g., 30% yield based on the L-aspartic acid initially present. The most common by-products are oligomers, 2-hydroxyethylamine N-succinic acid, and 2-bromoethylamine N-succinic acid.
The [S,S] EDDS salt produced by the Neal and Rose process is soluble in the reaction solution. To recover the salt from the solution, Neal and Rose teach that the reaction solution must be slowly acidified by adding concentrated hydrochloric acid to the solution to obtain a solution pH of 3.5. The acidification converts the [S,S] EDDS salt to [S,S] EDDS which crystallizes and precipitates from the solution. Fine crystals are said to precipitate out as the pH moves between pH 7 and 3.5. To purify the [S,S] EDDS precipitate, the precipitate is recovered and redissolved in a NaOH solution followed by reacidification. The cycle is repeated two times. The final precipitate is washed with water to remove HCl and any traces of L-aspartic acid.
While the Neal and Rose procedure may produce a relatively pure [S,S] EDDS product, it is burdened by (1) a high consumption of L-aspartic acid but with a low selectivity to obtain a low yield of recovered [S,S] EDDS (Neal and Rose reports the [S,S] EDDS yield at 25%), and (2) long process time and high HCl utilization, both due to the reacidification and redissolution cycles.